Thermal transmittance U of a building envelope element
Choose the element type and its layers, from outside to inside. The resistance of each layer, the surface resistances and the U value are calculated with the same functions the ZEPHREN calculation software uses for the envelope when it issues the certificate.
Indicative calculation; the official document is issued in the calculation software. The result is not a certificate and does not replace the envelope calculation in the certificate: thermal bridges, the corrections for existing buildings and heat transfer through the ground are handled there. The calculation runs in your browser and the data you enter is not saved.
The element
Layers, from outside to inside
Layer 1
λ = 0.870 W/(m·K) · source: C 107/3-2005 Anexa A poz. 16 (grupa IV Mortar de ciment și var, ρ=1700 kg/m³, p. 101)
Layer 2
λ = 0.044 W/(m·K) · source: C 107/3-2005 Anexa A poz. 72 (grupa XV Polistiren celular, ρ=20 kg/m³, p. 105)
Layer 3
λ = 0.800 W/(m·K) · source: C 107/3-2005 Anexa A poz. 63 (grupa XIII Zidărie din cărămizi pline, ρ=1800 kg/m³, p. 104)
Layer 4
λ = 0.870 W/(m·K) · source: C 107/3-2005 Anexa A poz. 16 (grupa IV Mortar de ciment și var, ρ=1700 kg/m³, p. 101)
Result
U = 0.341 W/(m²·K)
Total thermal resistance RT = 2.937 m²·K/W · U without the correction for fixings = 0.340 W/(m²·K)
Above the recommended value U′max = 0.25 W/(m²·K) (Mc 001-2022, Tab. 2.4 — clădiri rezidențiale nZEB noi, p. 60).
The U′max value in Mc 001-2022 is recommended, not mandatory: for nZEB the mandatory limits are the primary energy and emissions limits in Tab. 2.10a. It is compared with the corrected transmittance U′, which includes the thermal bridges of the element; the U here does not include them, so a U below this value is a necessary condition, not a sufficient one. The value shown is the one for new nZEB buildings; for major renovation, Tab. 2.9a and 2.9b are used.
Thermal resistances by layer and surface resistances
Layer
d (mm)
λ (W/(m·K))
R (m²·K/W)
External surface resistance RseMc 001-2022, Tab. 2.11, col. A — element în contact cu exteriorul, iarna (he = 24 W/(m²·K))
—
—
0.042
Tencuială var-cimentλ: C 107/3-2005 Anexa A poz. 16 (grupa IV Mortar de ciment și var, ρ=1700 kg/m³, p. 101)
10
0.870
0.011
Polistiren expandat EPS 80λ: C 107/3-2005 Anexa A poz. 72 (grupa XV Polistiren celular, ρ=20 kg/m³, p. 105)
100
0.044
2.273
Cărămidă plinăλ: C 107/3-2005 Anexa A poz. 63 (grupa XIII Zidărie din cărămizi pline, ρ=1800 kg/m³, p. 104)
375
0.800
0.469
Tencuială var-cimentλ: C 107/3-2005 Anexa A poz. 16 (grupa IV Mortar de ciment și var, ρ=1700 kg/m³, p. 101)
The table continues to the right: λ and R for each layer →
Correction for mechanical fixings ΔU″ = 0.0004 W/(m²·K) — ISO 6946:2017 Annex F (calcul exact). U = 1/RT + ΔU″.
What this calculation leaves out
Linear thermal bridges (ψ) — which is why the result is U, not U′ or R′. They depend on the details and lengths in the building.
Increase of conductivity in existing buildings (λ = a·λ, Mc 001-2022, rel. (2.3) and Tab. 2.2) — the λ here is the design value from the catalogue.
Weakly or strongly ventilated air layers — they are treated according to their position in the build-up (C 107/3-2005, Anexa E), so they are not offered here.
Heat transfer through the ground, for slabs and walls in contact with the soil.
Where the values come from
R per layer = d/λ and U = 1/RT: SR EN ISO 6946:2017.
Rsi and Rse: Mc 001-2022 (MDLPA Order 16/2023), Tab. 2.11, by direction of the heat flow and by what is on the outer side of the element.
λ: the source is given for every material. The catalogue here contains only materials whose λ comes from a published Romanian act (C 107/3-2005 Anexa A, Mc 001-2022) or from the manufacturer's documentation; the calculation software has the full catalogue.
ΔU″: SR EN ISO 6946:2017, Anexa F; it applies only when the build-up has an insulating layer within the meaning of C 107/2-2005, Tab. B.1, and the element has mechanically fixed insulation.
U′max: Mc 001-2022, Tab. 2.4 (new residential nZEB buildings) and Tab. 2.7 (new non-residential nZEB buildings).
The certificate is issued in the calculation software
In the calculation software, the same element goes into the building balance together with the thermal bridges, the windows and doors, the systems and the climate of the locality, and the result reaches the certificate, Annex 1 and Annex 2. The trial lasts 30 de zile, is free, needs no card, and asks for the MDLPA attestation number.